Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, classroom training, and for use in home theaters. In typical operation, multimedia projection systems receive analog video signals from a video unit and convert the video signals to digital information to control one or more digitally driven light modulators. Depending on the cost, brightness, and image quality goals of the particular projection systems, the light modulators may be of various sizes and resolutions, be transmissive or reflective, and be employed in single or multiple light path configurations.
Many light modulators used in projection and direct viewing systems operate on the basis of polarization. Such light modulators may include reflective or transmissive light valves based on liquid crystal technology. These liquid crystal light valves can produce a high-resolution image by changing the polarization state upon reflection or transmission of incident light. A polarization-analyzing device may then propagate the light from a bright state pixel of the light valve as a display image to be viewed by the human eye or projected onto a viewing screen.
There are several different optical architectures for employing liquid crystal light valves. One variation is a multipath optical architecture that provides a separate path for each of the primary color (red, blue, and green) lights. Polychromatic light is optically divided to provide each of the three pathways with its associated color light. The different color lights are routed through a series of polarization beam splitters, filters, and wave plates to a color-specific light valve. Each of the light valves is controlled with its respective color data in order to manipulate the colored light into image bearing light. The individual pathways are then reconverged into a color image. Although this design produces an acceptable image, the optics required for the color divergence, separate modulation, and reconvergence are expensive and costly to implement.
Another variation is a single-path multimedia projector. This type of projector involves only one light path that is sequentially illuminated with primary colors that time-share the same liquid crystal light valve. One alternative of this architecture employs a light source to produce polychromatic light rays, which are then directed through color filter segments of a color wheel. This filtering out of the nonselected color provides resource waste that could increase power consumption or decrease the brightness of the projected image.
Another alternative of the single-path multimedia projector employs monochromatic solid-state light sources, such as light-emitting diodes, to selectively emit the primary colors. This alternative requires expensive color combining optics in order to make the three color light beams coaxial before illuminating the liquid crystal light valve. Also, because the solid-state light sources are pulsed at 1/3 duty ratio the lumens/watt efficiency drops as the drive current increases.
A promising alternative to the above-mentioned optical architectures involves a microlens array that is optically coupled with the liquid crystal light valve. This microlens array receives the primary colored light along three ranges of incident angles and focuses the primary colors onto separate subpixels of each individual pixel. The subpixels can simultaneously and selectively modulate the individual colors in order to transmit a colored image. The promises of the microlens alternative, however, are tempered by inefficiencies due to light processing characteristics of prior art systems. These inefficiencies may result directly or indirectly from color separation, polarization filtering, and/or underfilling the subpixels.